A metal interconnect system in wide use in the later 1990's included an Al+Cu alloy interconnect line clad on each side with a barrier metal, and combined with planarized tungsten plugs for vias. A via is the structure that provides the electrical connection from one vertical level of interconnects to the next. The system saw wide acceptance in the industry, especially for high performance logic applications, such as microprocessor chips. The system was perceived as satisfactory, except that a severe degradation in electromigration resistance was noted on test patterns with multiple levels of interconnects and tungsten plug vias, versus test patterns using one interconnect level and no vias.
As much as a 100 times reduction in median-time-to-failure (T50) values, or more, were noted. One technical paper covering this phenomenon in detail is by R. G. Filippi et al., entitled, “The Effect of Copper Concentration on the Electromigration of Layered Aluminum-Copper (Ti-AlCu-Ti) Metallurgy With Tungsten Diffusion Barriers.” The paper appears in the 1992 VMIC Conference Proceedings, on page 359. The researchers showed that the copper doping is swept away from the tungsten in the direction of current flow. The aluminum, then depleted of its copper, electromigrates rapidly and voids appear at or near the W/Al interface. Increasing the concentration of copper helps to a limited extent, but degrades the resistivity. Stripes with a close by “reservoir” of copper also showed improvement, but none of these measures completely solved the problem. In general, the phenomenon may be referred to as a flux divergence at a dissimilar material interface.
A similar phenomenon has been noted in a copper system with tungsten plugs. This was reported, for example, by Kazuhide Abe, et al., and coworkers in a paper entitled, “Cu Damascene Interconnects with Crystallographic Texture Control and Its Electromigration Performance,” and appears in the IEEE 1998 Reliability Physics Symposium Proceedings on page 342.
The widely-accepted dual Damascene copper systems does not use tungsten plugs between interconnect levels, but does employ a barrier metal. This barrier layer lies, in general, between the upper surface of a copper interconnect and the bottom of an overlying copper via. Thus, some flux divergence may occur at this interface at high current density. The location of the copper metal depletion depends on the direction of current flow. For example, if the current flows up into overlying metal, this is the area of voiding and damage.